To design and discover new materials for next-generation energy materials such as solid-oxide fuel cells (SOFCs), a fundamental understanding of their ionic properties and behaviors is essential. The potential applicability of a material for SOFCs is
critically determined by the activation energy barrier of oxygen along various diffusion pathways. In this work, we investigate interstitial-oxygen (Oi) diffusion in brownmillerite oxide SrCoO2.5, employing a first-principles approach. Our calculations indicate highly anisotropic ionic diffusion pathways, which result from its anisotropic crystal structure. The one-dimensional-ordered oxygen vacancy channels are found to provide the easiest diffusion pathway with an activation energy barrier height of 0.62 eV. The directions perpendicular to the vacancy channels have higher energy barriers for Oint diffusion. In addition, we have studied migration barriers for oxygen vacancies that could be present as point defects within the material. This in turn could also facilitate the transport of oxygen. Interestingly, for oxygen vacancies, the lowest barrier height was found to occur within the octahedral layer with an energy of 0.82 eV. Our results imply that interstitial migration would be highly one-dimensional in nature. Oxygen vacancy transport, on the other hand, could preferentially occur in the two-dimensional octahedral plane.
We theoretically investigate the ground state magnetic properties of the brownmillerite phase of SrCoO2.5. Strong correlations within Co d electrons are treated within the local spin density approximations of Density Functional theory (DFT) with Hubb
ard U corrections (LSDAU) and results are compared with the Heyd Scuzeria Ernzerhof (HSE) functional. The parameters computed with a U value of 7.5 eV are found to match closely to those computed within the HSE functional. A G type antiferromagnetic structure is found to be the most stable one, consistent with experimental observation. By mapping the total energies of different magnetic configurations onto a Heisenberg Hamiltonian we compute the magnetic exchange interaction parameters, J, between the nearest neighbor Co atoms. The J s obtained are then used to compute the spin wave frequencies and inelastic neutron scattering intensities. Among four spin wave branches, the lowest energy mode was found to have the largest scattering intensity at the magnetic zone center, while the other modes becomes dominant at different momenta. These predictions can be tested by experimentally.
